fundamentals of the rf transmission and reception of digital signals

The World Leader in High-Performance Signal Processing Solutions

FUNDAMENTALS OF THE RF TRANSMISSION AND RECEPTION OF

DIGITAL SIGNALS

2


Part 1: Digital ModulationPart 1: Digital Modulation

3

Transmitting Bits

1 1 -1 1 1 -1 -1 -1

1 1 -1 1 1 -1 -1 -1

45 135 -45 -135

Bit Stream

45

-45

135

-135

1 1

11

45

Bits

Divide intoSymbols(2 bits per Symbol)

ModulatePhases on to

Carrier

Assign Phaseto Symbols

4

Practical Digital Modulation using an IQ Modulator

Phase Splitter separates LO from PLL into “Quadrature” components of equal amplitude but 90 degrees out of phase

Filtered bit streams from a dual DAC drive the I and Q inputs which are multiplied with the quadrature LOs

The outputs of the two multipliers are combined to yield the modulated carrier This modulation coding scheme is called Quadrature Phase Shift Keying

(QPSK)

90

0

Q IN

I IN

LO RF OUT

Looks like Amplitude Modulation (AM) but this signal is indeed phase modulated. Why the amplitude variations?

Filtered Bit Stream

Filtered Bit Stream

LO (from PLL)

5

IQ Modulation in the Frequency Domain

I and Q baseband signals are mixed up to an IF or to RF. Modulated carrier bandwidth is twice the baseband bandwidth

90

0

Q IN

I IN

LO RF OUT

FLO

3 dB BW=Symbol Rate/2

FLO

3 dB BW=Symbol Rate

6

Other Digital Phase Modulation Schemes

By allowing more I and Q levels (beyond -1 and +1), we can implement higher order QAM modulation schemes.

Higher Order Modulation Schemes Higher Data Rate. But Symbols are closer together Requires higher Signal-to-Noise Ratio for demodulation Increasing “Symbol Rate” increases data rate but widens Spectrum

8-PSK – 3 bits/symbol

m=8, n=3

16 QAM – 4 bits/symbol

m=16, n=4

64 QAM – 6 bits/symbol

m=64, n=8

QPSK- 2 bits/symbol

m=4, n=2

BPSK – 1 bit/symbol

m=2, n=1

7

Error Vector Magnitude - EVM

Noise and Imperfections in transmit and receive signal chains result in demodulated voltages which are displaced from their ideal location.

Error Vector Magnitude expresses this dislocation Large EVM will result in Symbol Errors and degraded Bit Error Rate Higher Order Modulation Schemes Symbols Closer Together EVM More

Critical

Ideal (Reference) Signal

Phase Error (I/Q error phase)

Magnitude Error (I/Q error mag)

{

I

Q

ActualSignal

M

k

M

k

kR

kRkZ

EVM

1

2

1

2

)(

)()(

Unit = %

8

The Imperfect IQ Modulator

IQ MOD

089.5

Vofs1

Vn

Vofs2

G1

G4

G3

G2

IIN

QIN

LOIN

Imbalance

In Phase

Splitter

Degrades

EVM

Gain

Imbalance

(G1,G2,G3,G4)

Degrades

EVM

Offset

Voltages

Cause LO

Leakage to

RFOUT

Noise risks

violation of

emissions

regulations

9

Dealing with IQ Modulator Imperfections

DAC incorporates Gain, Phase and Offset Voltage adjustment functions

DAC and IQ Modulator have matching bias levels (0.5 V), permitting a glue-less interface with no level shifting requirements

Modulator correction functions can also be performed in the digital domain

10

How Distortion Impacts Transmitters

A

Unit dBm

1RM

RBW 30 kHz

VBW 300 kHz

SWT 84 ms

Ref Lvl

-10 dBm

Ref Lvl

-10 dBm

RF Att 20 dB

3 MHz/Center 100 MHz Span 30 MHz

-100

-90

-80

-70

-60

-50

-40

-30

-20

-110

-101

Marker 1 [T1]

-10.73 dBm

99.48897796 MHz

1 [T1] -10.73 dBm

99.48897796 MHz

CH PWR 8.11 dBm

ACP Up -58.77 dB

ACP Low -59.27 dB

cu1cu1

cl1cl1

C0C0

Date: 24.FEB.2006 12:00:50

ACLR=58 dBc AdjacentChannelLeakageRatioCaused By poor IMD

No Blockers to worry about in Transmitter. But excessive distortion creates Spectral Leakage into adjacent

channels Distortion can be caused by any component in the signal chain, not just

the modulator

11

A

Unit dBm

RBW 10 kHz

VBW 100 kHz

SWT 370 ms

1RM

RF Att 0 dB

Ref Lvl

-30 dBm

Ref Lvl

-30 dBm

1.46848 MHz/Center 1.96 GHz Span 14.6848 MHz

1AVG

-120

-110

-100

-90

-80

-70

-60

-50

-40

-130

-30

1

Marker 1 [T1]

-79.38 dBm

1.95950000 GHz

1 [T1] -79.38 dBm

1.95950000 GHz

CH PWR -53.44 dBm

ACP Up -41.74 dB

ACP Low -41.71 dB

cu1cu1

cl1cl1

C0C0

Date: 9.NOV.2009 18:36:37

MAIN CHANNEL

ADJACENT CHANNEL

ADJACENT CHANNEL

12

A

Unit dBm

RBW 10 kHz

VBW 100 kHz

SWT 370 ms

1RM

RF Att 0 dB

Ref Lvl

-30 dBm

Ref Lvl

-30 dBm


1AVG

-120

-110

-100

-90

-80

-70

-60

-50

-40

-130

-30

1

Marker 1 [T1]

-60.22 dBm

1.95950000 GHz

1 [T1] -60.22 dBm

1.95950000 GHz

CH PWR -35.08 dBm

ACP Up -60.05 dB

ACP Low -60.01 dB

cu1cu1

cl1cl1

C0C0

Date: 9.NOV.2009 18:33:38

MAIN CHANNEL

ADJACENT CHANNEL

ADJACENT CHANNEL

13

A

Unit dBm

RBW 10 kHz

VBW 100 kHz

SWT 370 ms

1RM

RF Att 0 dB

Ref Lvl

-30 dBm

Ref Lvl

-30 dBm


1AVG

-120

-110

-100

-90

-80

-70

-60

-50

-40

-130

-30 1

Marker 1 [T1]

-33.52 dBm

1.95950000 GHz

1 [T1] -33.52 dBm

1.95950000 GHz

CH PWR -8.92 dBm

ACP Up -68.55 dB

ACP Low -71.69 dB

cu1cu1

cl1cl1

C0C0

Date: 9.NOV.2009 18:10:08

MAIN CHANNEL

ADJACENT CHANNEL

ADJACENT CHANNEL

14

A

Unit dBm

RBW 10 kHz

VBW 100 kHz

SWT 370 ms

1RM

RF Att 0 dB

Ref Lvl

-30 dBm

Ref Lvl

-30 dBm


1AVG

-120

-110

-100

-90

-80

-70

-60

-50

-40

-130

-30

1

Marker 1 [T1]

-42.87 dBm

1.95950000 GHz

1 [T1] -42.87 dBm

1.95950000 GHz

CH PWR -17.67 dBm

ACP Up -73.47 dB

ACP Low -74.75 dB

cu1cu1

cl1cl1

C0C0

Date: 9.NOV.2009 18:12:07

15

A

Unit dBm

RBW 10 kHz

VBW 100 kHz

SWT 370 ms

1RM

RF Att 0 dB

Ref Lvl

-30 dBm

Ref Lvl

-30 dBm


1AVG

-120

-110

-100

-90

-80

-70

-60

-50

-40

-130

-30

1

Marker 1 [T1]

-36.78 dBm

1.95950000 GHz

1 [T1] -36.78 dBm

1.95950000 GHz

CH PWR -11.53 dBm

ACP Up -72.85 dB

ACP Low -74.71 dB

cu1cu1

cl1cl1

C0C0

Date: 9.NOV.2009 19:14:23

16

A

Unit dBm

RBW 10 kHz

VBW 100 kHz

SWT 370 ms

1RM

RF Att 0 dB

Ref Lvl

-30 dBm

Ref Lvl

-30 dBm


1AVG

-120

-110

-100

-90

-80

-70

-60

-50

-40

-130

-30 1

Marker 1 [T1]

-33.52 dBm

1.95950000 GHz

1 [T1] -33.52 dBm

1.95950000 GHz

CH PWR -8.92 dBm

ACP Up -68.55 dB

ACP Low -71.69 dB

cu1cu1

cl1cl1

C0C0

Date: 9.NOV.2009 18:10:08

17

What is happening here?

OIP3 Intercept(dBm) = PFUND – (IMD/2) Knowing the OIP3 allows you to calculate Intermodulation Distortion (IMD) at any power

level Many devices do not follow this rule

3020100-10-20

*

50

0

-50

-100

-1505040

****

*****

*SLOPE=1

SLOPE=3

Fundamentals

Intermods

Interceptof

Fundamentalsand

Intermods(IP3)

IMD(dBc)

18

Striking a Balance

A

Unit dBm

RBW 10 kHz

VBW 100 kHz

SWT 370 ms

1RM

RF Att 0 dB

Ref Lvl

-30 dBm

Ref Lvl

-30 dBm


1AVG

-120

-110

-100

-90

-80

-70

-60

-50

-40

-130

-30

1

Marker 1 [T1]

-79.38 dBm

1.95950000 GHz

1 [T1] -79.38 dBm

1.95950000 GHz

CH PWR -53.44 dBm

ACP Up -41.74 dB

ACP Low -41.71 dB

cu1cu1

cl1cl1

C0C0

Date: 9.NOV.2009 18:36:37

A

Unit dBm

RBW 10 kHz

VBW 100 kHz

SWT 370 ms

1RM

RF Att 0 dB

Ref Lvl

-30 dBm

Ref Lvl

-30 dBm


1AVG

-120

-110

-100

-90

-80

-70

-60

-50

-40

-130

-30 1

Marker 1 [T1]

-33.52 dBm

1.95950000 GHz

1 [T1] -33.52 dBm

1.95950000 GHz

CH PWR -8.92 dBm

ACP Up -68.55 dB

ACP Low -71.69 dB

cu1cu1

cl1cl1

C0C0

Date: 9.NOV.2009 18:10:08

We need to set our gains and levels so that we can strike a balance between SNR and Distortion

This is why our customers simultaneously demand low noise and low distortion

Gain is generally distributed throughout the channel to achieve this goal

Poor SNR Excessive Distortion

19

A

Unit dBm

RBW 10 kHz

VBW 100 kHz

SWT 370 ms

1RM

RF Att 0 dB

Ref Lvl

-30 dBm

Ref Lvl

-30 dBm


1AVG

-120

-110

-100

-90

-80

-70

-60

-50

-40

-130

-30

1

Marker 1 [T1]

-42.87 dBm

1.95950000 GHz

1 [T1] -42.87 dBm

1.95950000 GHz

CH PWR -17.67 dBm

ACP Up -73.47 dB

ACP Low -74.75 dB

cu1cu1

cl1cl1

C0C0

Date: 9.NOV.2009 18:12:07

Last Word on Distortion…..

SpuriousFreeDynamicRange

•During an IP3 sweep, at a certain power level, the power of the IMD tones will be equal to the noise power in a defined bandwidth. The SNR at this point is the SFDR of the component•Don’t mix this up with the SFDR of an ADC or DAC

SFDR = (2/3)(IIP3-NF-10log(kTB))

20

Key IQ Modulator Specifications

Input IP3 (IIP3): Same as OIP3 but referred to input: Intermodulating Blockers can create IMD products that fall on the desired signal

Noise Figure IP2: Figure of Merit for Second order Intermodulation

Distortion. Poor IP2 can intermodulate with the desired signal and produce dc offsets

LO Quadrature accuracy: Affects EVM/BER of recovered data

21

I/Q Modulator Key specificationsPart

NumberFreq

(MHz)Desc

LO(dBm)

Sideband(dBc)

Noise(dBm/Hz)

P1dB(dBm)

OIP3(dBm)

Specs @ (MHz)

P/NdBc/Hz

Vs(V)Isy

(mA)Package

AD8345 140-1000 Low Power I/Q Mod -42 -42 -154.5 2.5 25 800 N/A 2.7-5.5 655.1×6.4

TSSOP-16

AD8346 800-2500 Low Power I/Q Mod -42 -36 -147 -3 20 1900 N/A 2.7-5.5 455.1×6.4

TSSOP-16

AD8349 700-2700 Low Power I/Q Mod -45 -35 -155 7.6 21 900 N/A 4.75-5.5 1355.1x6.4

TSSOP-16

ADF9010 840-960 IQ Mod & Int-N PLL -40 -46 -158 10 24 900 -83 3.15-3.45 3607X7

LFCSP-48

ADL5370 300-1000 Narrowband IQ Mod -50 -41 -160 11.0 24 450 N/A 4.75-5.25 2054×4

LFCSP-24

ADL5371 500-1500 Narrowband IQ Mod -50 -55 -158.6 14.4 27 900 N/A 4.75-5.25 1754×4

LFCSP-24

ADL5372 1500-2500 Narrowband IQ Mod -45 -45 -158 14.2 27 1900 N/A 4.75-5.25 1654×4

LFCSP-24

ADL5373 2300-3000 Narrowband IQ Mod -32 -57 -157.1 13.8 26 2500 N/A 4.75-5.25 1744x4

LFCSP-24

ADL5374 3000-4000 Narrowband IQ Mod -32.8 -50 -159.6 12.0 22.8 3500 N/A 4.75-5.25 1734×4

LFCSP-24

ADL5375 400-6000 IQ Mod w Output Disable -46.2 -52.1 -160 9.4 26.8 900 N/A 4.75-5.25 2004×4

LFCSP-24

ADL5385 50-2200 2XLO Broadband IQ Mod -46 -50 -159 11.0 26 350 N/A 4.75-5.5 2154×4

LFCSP-24

ADL5386 50-2200 2XLO IQ Mod & VVA&AGC -38 -46 -160 11.1 25 350 N/A 4.75-5.5 2306×6

LFCSP-40

ADRF6701 750-1100 IQ Mod & Frac-N PLL&VCO -45 -40 -158 14 29 900 -93 4.75-5.25 2606x6

LFCSP-40


LFCSP-40


LFCSP-40


LFCSP-40

ADRF6750 950-1575 IQ Mod & Frac-N PLL&VCO -45 -45 -157 8.5 21 1200 -93 4.75-5.25 3108X8

LFCSP-56

22


Part 2: Digital DemodulationPart 2: Digital Demodulation

23

Recovering Data from a Digitally Modulated Carrier

Iout

Qout

Reverse process to IQ Modulation IQ Demodulator extracts phase (and amplitude) information from

the modulated signal and presents it in XY (or IQ) format. Apply I and Q outputs to an ADC or Comparator and bits can be

recovered.

0

90

70 MHzSine Wave

70 MHz

VREF

VREF

Comparators(real systems use Dual ADCs)

24

Critical IQ Demodulator Specs – LO to RF Leakage

•If some of the LO leaks to the RF input, it mixes (multiplies) with itself in the mixer generating unwanted dc offsets on top of the recovered baseband data stream

ADCLNA

Desired

-70dBm

0dBm

Leakage

-60dBm

-40dBm

-30dBm(~20mVp-p)

A B C

Assume,

Gain from A to C =30dB

LO to RF leakage ~ 60dBFLO

FLO

25

What is causing the poor quality of this demodulated Constellation?

Very poor LO Quadrature Phase Split (in DMOD) Dc Offset of the complete constellation (probably LO to RF Leakage) Noise has enlarged the footprint of the constellation points (poor Receiver Noise Figure)

SymbolDecision

ThresholdIf the symbol lands

on the edge or outsideof the box, bit errors

will occur

Reading the Demodulated Constellation

Signal Compression (signal chain is being over driven)

27

Key IQ DMOD Specifications

Input IP3 (IIP3): Same as OIP3 but referred to input: Intermodulating Blockers can create IMD products that fall on the desired signal

Noise Figure IP2: Figure of Merit for Second order Intermodulation

Distortion. Poor IP2 can intermodulate with the desired signal and produce dc offsets

LO Quadrature accuracy: Affects EVM/BER of recovered data

28

IQ Demodulators

Part No.Freq

(MHz)

VGA Range(dB)

IQ 3dB BW(MHz)

Quadrature Error

(dB/deg)

P1dB(dBm)

IIP3(dBm)

NoiseFigure(dBm)

Specs@(MHz)

Isy(mA)

VS

(V)Package

AD8347 800-2700 70 65 0.3/1º -2 +11.5 11 1900 64 2.7-5.59.7x6.4

TSSOP-28

AD8348 50-1000 44 125 0.25/0.5º +13 +28 10.75 380 48 2.7-5.59.7x6.4

TSSOP-28

ADL5382 700-2700 N/A 370 0.05/0.2º 14.4 30.5 15.6 1900 220 4.75-5.254X4

LFCSP-24

ADL5387 50-2000 N/A 240 0.05/0.2º +13 +31 12 140 180 54X4

LFCSP-24

ADL5380 400-6000 N/A 390 0.07/0.25º 11.6 27.8 11.7 1900 245 54X4

LFCSP-24

ADRF6850 100-1000 60 300 0.1/0.5º 12 22.5 11 800 350 3.15-3.458X8

LFCSP-56

29

Application Example – Complete Direct Conversion Receiver

Direct Conversion Receiver has no IFs and no IF Filters

Variable gain after IQ DMOD is used to optimize the peak-to-peak swing of the signal for the ADCs

30

Receiver EVM vs Input power

using ADF4350 PLL/VCO as LO source

-40

-35

-30

-25

-20

-15

-10

-90 -80 -70 -60 -50 -40 -30 -20Input Power (dBm)

Mod

ulati

on E

rror

Rat

e-M

ER-d

B

using ADF4350PLL/VCO as LOsource

31

An IQ DMOD-based Receiver

Filters and Amplifiers amplify signal and remove out-of-band blockers

Variable gain after IQ DMOD is used to optimize the peak-to-peak swing of the signal for the ADCs

When the input frequency to the IQ Modulator is also the receive frequency, we have a Direct Conversion Receiver (Zero IF)

32

AD8348 IQ Demodulator with Integrated VGA

Built-in VGA has 45 dB of gain control range VGA will still require external circuitry to implement AGC

fundamentals of the rf transmission and reception of digital signals

Technology